1. Field of the Invention
The present invention relates to a snapshot technique for a shared storage supporting large capacity, and more particularly, to a snapshot technique which supports an on-line backup for a large logical volume based on a storage area network (SAN).
2. Discussion of the Related Art
In recent years, as technical developments of Internet-based application such as an electronic commerce and the like are made and users of Internet are rapidly spreading, a quantity of data in service increases exponentially. For this reason, there are demands for large network storage which can effectively share a large quantity of information and provide service at high speed.
As an example of technologies for implementing the large network storage, a network attached storage (NAS) and a storage area network (SAN) have been proposed. The SAN is a data file-oriented computer system environment which can directly access to a storage connected to a network, not via a server.
In the meantime, enterprise systems that must support 24×7×365 environment require data availability and reliability as well as high-speed processing of large-sized data. Among several methods of ensuring the reliability and availability, one method that can meet the requirements of these systems is an on-line backup whose importance has been stressed. A backup execution time increases exponentially in order for a backup of a large-sized data. Therefore, it is essential to provide an on-line snapshot based on a mapping table, since a system which stops its operation in order to execute the backup and then resumes its service is not useful. Here, the snapshot is a technique for storing and retaining data state at specific time when a user wants. The snapshot is a useful technique for the on-line backup and the like.
The snapshot technique copies only data image, not entire data, and retains data obtained at the moment the snapshot is created. If data block is modified after the snapshot is created, a new block is allocated, and then, the data at the moment of the snapshot is copied. Thereafter, mapping entry values are changed so as to map a data block which is newly allocated. In other words, a copy-on-write (COW) operation is performed in order to retain the data obtained at the time of the snapshot creation.
However, when the snapshot creation request is carried out in the on-line snapshot based on a conventional mapping table, a service cannot be processed because all hosts' access to an original volume is disconnected while copying the snapshot mapping table. As a size of the volume becomes larger, the mapping table increases. As a result, I/O access protection time also increases proportionally.
In addition, the write operation of data block occurring after the COW operation requires many disk I/O operations, thereby degrading I/O performance of the volume. At a snapshot destruction operation, in order for the deallocation of the data block allocated by the COW operation, it is checked whether the data block of the original volume and the data block of snapshot volume are updated or not, and the newly allocated data block should be deallocated, thus increasing the snapshot destruction execution time.